This invention relates to cryogenic filters and, in particular, to systems and methods for use in implementing cryogenic filters in wireless communication systems.
As demonstrated by the high prices paid for licenses to portions of the radio frequency spectrum in the United States, there is a need to improve the services that can be provided over a limited bandwidth. This need is particularly critical in the field of cellular phone communication systems. For example, as the self-induced noise of the filter is decreased, the sensitivity of the receiver is increased. Accordingly, the range of the base station may be increased--reducing the number of required base stations. Additionally, as the cut-off characteristics of the filter become sharper, co-channel interference is reduced.
One technique for improving filter response is to utilize high temperature super-conducting (HTS) waveguide filters/amplifiers which operate up to a temperature of 77 degrees Kelvin. However, these filters are not well adapted to mass commercial implementations. HTS cooling systems capable of reaching a temperature of 77 degrees Kelvin suffer from persistent problems of high cost and low reliability. Although HTS filters can be configured to offer low noise characteristics, they nonetheless suffer from a lack of a sharp cut-off characteristic (as compared to elliptical response filters) and temperature instability.
Conventionally, a superconducting filter/LNA operates at temperatures of 77 Kelvin or below. The receive filters are conventionally constructed using either thin film or thick film deposition approaches. In most instances, the LNA is not made of any superconducting materials. The reason for operating both devices at such low temperatures is related to the sharp transition of the loss characteristics of the superconductor. As a result, the unloaded "Q" of the resonator is found to be in order of tens of thousands.
In the case of thin film technology, the filter utilizes a micro-strip media. This implies an elaborate design, which requires extremely accurate models. Often a 3-D simulation is used, to compensate and optimize for the unequal phase velocities of the Even and Odd modes. It is very difficult, by any means, to achieve an elliptical response. The thin film approach is well suited for "roofing" filters, wide pass-bands and low number of sections. Due to the limited rejection, this type of filter is not suitable for co-channel interference. The major advantages of micro-strip filters are miniaturization (in respect to other techniques) and reproducibility.
Thick film technology is based on the idea that TEM resonators can be coated with a superconducting material and cooled. Again cooling at an operating temperature of 77 Kelvin has sever cost and reliability problems. With thick film technology, the size of the filter is closer to a conventional cavity than to micro-strip (thin film) filters. The thick film filters are also problematic in that they exhibit non-linearity and have limited power handling capabilities.
Another method is to coat metallic loaded wave-guide structures with superconducting films. This approach can realize Dual-Mode propagation, which can save a number of resonators, but is extremely expensive and suffers from reliability problems.
In conventional architectures, the filter/amplifier module is physically placed in the base-station, and is connected with a long low-loss coaxial cable to the tower location where the antennas are mounted. The base station mounted filter is usually made of metal cavities, with typical insertion loss of -1.5 dB.
Due to the extreme reliability problems with HTS cooling systems and the fact that HTS filters will not operate when the cooling system is broken, it is usually necessary to construct an entirely redundant path whereby the system can bypass a broken cooling system. Further, tower mounted HTS cooling systems are not practical due to the reliability problems with cooling to HTS temperatures.
Superconductor substrates for HTS filters cost $1,200 even for a small surface areas. Accordingly, the superconducting substrate is limited to one by one or by two by two inches. Because the substrate is mounted on the metal, thermal coefficients of expansion dictate that the metal be a special tungsten copper alloy which is very heavy.
Additionally, for HTS devices, the filters become totally non-functional with a cooling malfunction since the devices cease to be super conductive. Accordingly, these HTS systems are expensive and require complicated redundancy systems.